simplewallet: allow setting confirm-missing-payment-id in watch wallets

These can create transactions, even though they cannot sign them.

src/simplewallet/simplewallet.cpp

@@ -559,12 +559,6 @@ bool simple_wallet::set_refresh_type(const std::vector<std::string> &args/* = st
 
 bool simple_wallet::set_confirm_missing_payment_id(const std::vector<std::string> &args/* = std::vector<std::string>()*/)
 {
-  if (m_wallet->watch_only())
-  {
-    fail_msg_writer() << tr("wallet is watch-only and cannot transfer");
-    return true;
-  }
-
   const auto pwd_container = get_and_verify_password();
   if (pwd_container)
   {